1. Field of the Invention
The present invention relates to a multilayer laminate endowed with excellent antireflective properties and mar resistance which is obtained by laminating onto a transparent substrate, in order: a protective layer, a high refractive index layer, and a low refractive index layer.
2. Prior Art
Displays for electronic equipment such as personal computers, word processors, televisions and plasma displays make use of glass or, when a lighter weight material is called for, transparent plastic. During the observation and recognition of visual information such as writing, graphics and objects through such a transparent substrate, visual information at the interior is often difficult to see because of outside light reflected by the surface of the substrate.
One method used to prevent reflection by such a transparent substrate is to provide a single low refractive index layer on the surface of the transparent substrate, which is typically a sheet of glass or a plastic substrate (e.g., plastic lens, plastic film, plastic sheet). Unfortunately, the difference in refractive index achieved in this way is too small to provide a satisfactory antireflective effect. Such a low refractive index layer can be formed by vapor-depositing an inorganic metal oxide. This process provides a good coat, but the equipment needed to carry it out is very expensive to install and the process itself imposes limitations on the size of the substrate. A need has thus been felt for a liquid coating-based treatment that is relatively free of such impediments.
Various coatings developed to avoid the drawbacks of a vapor deposition process have had mixed success. For example, methods that involve coating a fluoropolymer-containing solution (JP-A 2-19801, JP-A 10-147740) resolve the problems associated with vapor deposition, but the poor adhesion of such fluoropolymers to the substrate and the inadequate hardness of the cured film have resulted in a poor resistance to marring.
JP-A 63-21601 describes a method for coating the hydrolyzate of a perfluoroalkylsilane compound, but uniform hydrolysis and a coat of uniform transparency are difficult or impossible to achieve using long-chain fluorosilanes. Moreover, use of the short-chain trifluoropropylsilane mentioned in the examples given in the specification fails to lower the refractive index to a sufficient degree. Hence, both the antireflective properties and the mar resistance are inadequate.
JP-A 2000-171604 describes a method in which an alkylsilane compound is cured with a photoacid generator, but here too the antireflective properties are inadequate. Hence, all of the above prior-art methods fail to achieve a sufficient antireflective effect using a low refractive index layer alone, and also fail to provide adequate mar resistance.
A number of attempts have been made to improve the antireflective properties by laminating a high refractive index layer and a low refractive index layer onto a transparent substrate. For example, JP-A 9-288202 describes an antireflection system composed of a high refractive index layer containing a metal alkoxide and a colloidal metal oxide in combination with a noncrystalline fluorocarbon resin-based low refractive index layer. Although this system achieves an acceptable degree of antireflective effects, the low refractive index layer does not adhere well to the high refractive index layer and has insufficient surface hardness, resulting in a poor mar resistance.
JP-A 9-226062 discloses a system which vacuum evaporates a low refractive index layer composed of SiO2 onto a high refractive index layer composed of a reactive organosilicon compound-containing ionizing radiation-curable resin and fine particles having a high refractive index. However, this system requires the use of a vapor deposition apparatus and thus entails excessive costs. JP-A 2000-117902 describes a system in which the low refractive index layer is composed instead of the hydrolyzate of a long-chain fluorosilane, but this has an inadequate mar resistance. In another system, described in JP-A 2000-198964, the hydrolyzate of a hydrolyzable silane serves as the low refractive index layer. However, the layer does not have a sufficiently low refractive index. Moreover, neither the antireflective effects nor the mar resistance are adequate. Given the principles involved in preventing reflection, a good antireflective effect generally requires that the film thickness of the high refractive index layer and the low refractive index layer be controlled to about ¼ λ (where λ is the wavelength of the incident light), and thus substantially to about 0.1 μm. Yet, films with a thickness of this order have inherent limitations in that they cannot withstand marring forces and thus tend to become marred.
To further improve mar resistance, methods have been proposed for successively laminating a hard protective coat, a high refractive index layer and a low refractive index layer. In one such process, disclosed in JP-A 9-96702, a high refractive index layer composed primarily of a metal alkoxide is formed on a hard coat composed primarily of an acrylic resin or an organosilicon compound in combination with a colloidal oxide, and a low refractive index layer composed primarily of an organosilicon compound and a colloidal oxide is formed on the high refractive index layer. This arrangement provides a substantially improved mar resistance, but the high refractive index layer is still not hard enough. In addition, the low refractive index layer does not have a low enough refractive index, as a result of which the antireflective properties also are inadequate.
Hence, a transparent substrate coating process which provides both good antireflective properties and mar resistance has not previously been found.